1. Field of the Invention
The present invention relates to a technology for transmitting image signals obtained by encoding image information which utilizes the so-called vector quantization technique and which is applicable to the fields such as the television (TV) conference system and the TV telephone system.
2. Description of the Prior Art
As a result of the remarkable advance of the image processing technology in recent years, there have been made various attempts to put, for example, the TV conference system and the TV telephone system to the practical use by mutually and bidirectionally transmitting the image information. In such a technological field, the quantization technique has been used in which the image signals as the analog quantity are classified into a finite number of levels changing in a discrete fashion within a fixed width and a unique value is assigned to each of these levels. Particularly, there has been a considerable advance in the vector quantization technique in which a plurality of samples of the image signals are grouped in blocks and each block thereof is mapped onto a pattern most similar thereto in a multidimensional signal space; thereby accomplishing the quantization.
The study of the vector quantization technology has been described in the following reference materials, for example.
(1) "An Algorithm for Vector Quantizer Design" by Y. Linde, A. Buzo, and R. M. Gray (IEEE TRANSACTION ON COMMUNICATIONS, Vol. COM. 28, No. 1, Jan. 1980, pp. 84-95) PA0 (2) "On the Structure of Vector Quantizers" by A. Gersho (IEEE TRANSACTION ON INFORMATION THEORY, Vol. IT28, No. 2, Mar. 1982, pp. 157-166) PA0 (3) "Speech Coding Based Upon Vector Quantization" by A. Buzo, A. H. Gray Jr., R. M. Gray and J. D. Markel (IEEE TRANSACTION ON ACOUSTICS, SPEECH, AND SIGNAL PROCESSING, Vol. ASSP28, No. 5, Oct. 1980, pp. 562-574) PA0 (4) U.S. Pat. No. 4,558,350 "VECTOR QUANTIZER", Murakami PA0 (5) U.S. Pat. No. 4,560,977 "VECTOR QUANTIZER", Murakami et al. PA0 (6) U.S. application Ser. No. 819,067 "VIDEO ENCODING APPARATUS", Kubo et al, filed on Jan. 15, 1986.
Moreover, the following U.S. Patents have been obtained by the assignee of the present invention.
An image encoding apparatuses to which the vector quantization technology described in the reference materials above is applied include the following device.
Referring now to FIGS. 1-3B, the prior art technology of the present invention will be described. The conventional image encoding/transmitting apparatus, as shown in FIG. 1, includes a subtractor 1 for obtaining a difference between an input signal S.sub.1 such as an image signal and an estimation signal S.sub.9 and for outputting an estimated error signal S.sub.2, a movement detecting circuit 2 for comparing a threshold value T with the estimated error signal S.sub.2 to detect a movement or a change and for generating and outputting a movement or change detect signal S.sub.3 and a differential signal S.sub.4, a quantization circuit 3 for quantizing the movement or change detect signal S.sub.3 and the differential signal S.sub.4 to output a quantization signal S.sub.5, a variable length encoder 4 for generating from the quantization signal S.sub.5 an encoded signal S.sub.6 with a variable length and for outputting the encoded signal S.sub.6, a transmission data buffer circuit 5 for temporarily storing the encoded signal S.sub.6 and for outputting the encoded signal S.sub.6 to the transmission side, a local decoding circuit 6 for generating a reproduced differential signal S.sub.7 from the quantization signal S.sub.5 delivered from the quantization circuit 3 and outputting the reproduced or regenerated differential signal S.sub.7, an adder 7 for achieving an addition on the reproduced differential signal S.sub.7 and the estimation signal S.sub.9 and for outputting a reproduced input signal S.sub.8, an estimating circuit 8 for outputting an estimation signal S.sub.9 based on the reproduced input signal S.sub.8, and a threshold generating circuit 9 for monitoring the amount of the encoded signal S.sub.6 accumulated in the transmission data buffer circuit 5 and for generating an appropriate threshold value T.
The movement detecting circuit 2 comprises, as shown in FIG. 2, an absolute value circuit 10 for calculating the absolute value .vertline.S.sub.2 .vertline. of the estimated error signal S.sub.2, a comparing circuit 11 for effecting a comparison between the absolute value .vertline.S.sub.2 .vertline. of the estimated error signal S.sub.2 and the threshold value T and for outputting the movement or change detect signal S.sub.3, and a zero allocator 12 for allotting 0 and outputting 0 as the differential signal S.sub.4 when the movement or change is not detected as a result of the comparison in the comparing circuit 11. The movement detect signal S.sub.3 is converted into a running record R by use of the running length encode table 4a to generate serial data. In addition, only when the movement detect signal S.sub.3 is indicating the validness, the quantization signal S.sub.5 is converted into a variable-length record through the variable-length encoding table 4b to generate serial data (FIGS. 2B-2C). Reference numeral 4c indicates a multiplex operation control section.
In contrast to the configuration on the transmission side of FIGS. 1-2B, the configuration on the reception side is shown in FIGS. 3A-3B. In FIG. 3A, the equipment on the reception side includes a receiving data buffer circuit 13 for receiving and for temporarily storing the encoded signal S.sub.6 delivered from the transmission data buffer circuit 5 on the transmission side, a variable length decoder 14 for decoding the encoded signal S.sub.6 stored in the receiving data buffer 13 to output a reproduced quantization signal S.sub.11, a local decoding circuit 15 for outputting a reproduced differential signal S.sub.12 based on the reproduced quantization signal S.sub.11, an adder circuit 16 for obtaining the sum of the reproduced differential signal S.sub.12 and the reproduced estimation signal S.sub.13 and for reproducing the input signal S.sub.14 which corresponds to the reproduced input signal S.sub.8 on the transmission side, and an estimating circuit 17 for outputting the reproduced estimation signal S.sub.13.
After the encoded signal S.sub.6 has undergone the multiplexing in the variable-length encode circuit 4 is received by the receiving buffer circuit 13, the data is distributed to the respective decode tables of variable codes under control of the multiplex separation control circuit 14a. As a result of the decoding, the movement detect signal and the quantization signal are attained. Moreover, when the decoded movement detect signal indicates the invalidness (="0"), the quantization signal is reset to ".0." by the flip-flop 14e, thereby outputting the output S.sub.11 (FIG. 3B).
Next, the operation on the transmission side will be described with reference to FIGS. 1-2.
Assuming first the non-effective error in the movement detecting circuit 2 to be d, the estimation coefficient to be applied to the reproduced input signal S.sub.8 in the estimating circuit 8 to be A, and the delay of the time t to be Z.sup.-t, the following relationships are satisfied. EQU S.sub.2 =S.sub.1 -S.sub.9 EQU S.sub.4 =S.sub.2 +d EQU S.sub.7 =S.sub.4 +Q EQU S.sub.8 =S.sub.7 +S.sub.9 =S.sub.1 +Q+d EQU S.sub.9 =A.multidot.S.sub.8 .multidot.Z.sup.-t
The subtractor 1 calculates the estimated error signal S.sub.2 representing the difference between the input signal S.sub.1 and the estimated signal S.sub.9, whereas the movement detecting circuit 2 outputs the movement or change detection signal S.sub.3 and the differential signal S.sub.4 based on the estimated error signal S.sub.2 calculated by the subtractor 1.
A detailed description will be given of the operation of the movement detecting circuit 2 by referring to FIG. 2. The allotting absolute value circuit 10 obtains the absolute value of the estimated error signal S.sub.2 and then the comparison circuit 11 achieves a comparison between the absolute value .vertline.S.sub.2 .vertline. of the estimated error signal S.sub.2 and the threshold value T generated by the threshold value generating circuit 9.
The movement detection signal S.sub.3 is output in conformity with the following conditions. EQU S.sub.3 =0 (invalid),.vertline.S.sub.2 .vertline.&lt;T EQU S.sub.3 =1 (valid), .vertline.S.sub.2 .vertline..gtoreq.T
When the movement or change is not detected, namely, for "S.sub.3 =0", zero allocator 12 outputs "0" for the differential signal S.sub.4.
On the other hand, the quantization circuit 3 converts the inputted differential signal S.sub.4 according to an arbitrary characteristic. The variable encoding circuit 4 receives the quantization signal S.sub.5 only when the movement detection signal S.sub.3 is valid, namely, for "S.sub.3 =1" and, for example, conducts a run-length encoding on the movement detection signal S.sub.3. For the quantization signal S.sub.5, a code having a smaller code length is assigned to a value in the neighborhood of "0" for which the generation frequency is high and then the code is stored in the transmission data buffer circuit 5. The transmission data buffer circuit 5 outputs the accumulated datum as the encoded signal S.sub.6 to a transmission line. The threshold generating circuit 9 monitors the accumulated amount of the transmission data buffer circuit 5 and further controls the generation amount of the encoded data by generating an appropriate threshold value.
Next, the operation on the reception side will be described with reference to FIG. 3. The receiving data buffer circuit 13 first receives the encoded signal S.sub.6 which has undergone the variable length encoding on the transmission signal and outputs the signal S.sub.6 to the variable length decoder 14. Only when the movement detection signal S.sub.3 having undergone the decoding operation indicates the validness, the variable length decoder 14 outputs the reproduced quantization signal S.sub.11. If the movement detection signal S.sub.3 indicates the invalidness, the variable length decoder 14 outputs "0". Next, the local decoding circuit 15 decodes the reproduced quantization signal S.sub.11 and outputs the reproduced differential signal S.sub.12 to the adder 16. The adder 16 adds the reproduced differential signal S.sub.12 to the reproduced estimation signal S.sub.13 from the estimation circuit 17 thereby to reproduce the input signal S.sub.14.
The operation to effect the data compression and transmission by use of the differential signal is referred to as the differential pulse code modulation (to be abbreviated as DPCM herebelow) system.
However, in the image encoding/transmitting apparatus using the DPCM system, the variable length encoding is achieved on the datum which is judged to be effective at the step of the variable length encoding; consequently, as the threshold value increases, the code having a short code length to be assigned in the neighborhood of "0" cannot be generated and hence the efficiency of the encoding is deteriorated; moreover, there has been a problem that as the threshold value becomes greater, the precision of the quantization is not improved for the quantization characteristic of the quantization circuit in the circuitry on the transmission side even when the dynamic range of the effective datum is narrowed.